DE69325103T2 - SCARF WITH IMPROVED THICKNESS STABILITY WHEN USED - Google Patents

Description

Background of the invention

This invention relates to a printing blanket and, more particularly, to a printing blanket having improved dynamic thickness stability and resistance to permanent dimensional loss.

In offset lithography, a rotating cylinder is covered with a printing plate, which usually has a positive image area receptive to oil-based inks and repellant to water, and a background area where the opposite is the case. The printing plate is rotated so that its surface contacts a second cylinder, which is covered with a rubber-surfaced, ink-receptive printing blanket (sometimes called the printer's blanket). The ink present on the image surface of the printing plate transfers, or offset prints, onto the surface of the blanket. Paper or other sheet material to be printed on is then passed between the blanket-covered cylinder and a rigid counter cylinder to transfer the image from the surface of the blanket to the paper.

During the step of transferring the inked image from the plate to the blanket and the step of transferring the image from the blanket to the paper, it is important to have intimate contact between the two contact surfaces. A precise amount of intermediate pressure is required so that the blanket contacts the image plate and picks up ink from it and transfers the inked image into the paper at a suitable depth. This is usually achieved by positioning the blanket-covered cylinder and the support cylinder it contacts so that there is a fixed interference between the two and so compresses the blanket to a fixed depth, typically about 0.05 to 0.10 mm (0.002 to 0.004 inches), throughout the run. It is important that this compression be maintained uniformly over the entire surface of the blanket.

In the current state of the art, all printing blankets lose thickness (i.e., lose their dimensional stability or "sag") when they are initially tensioned and installed, and continue to lose thickness as the blanket is repeatedly subjected to interference pressures at the nips between the impression cylinder and the blanket-covered cylinder and the rigid counter cylinder, respectively. Blankets can fail catastrophically due to fabric breakage, permanent deformation in an area of the entire blanket surface, or from gradual deterioration of the blanket dimensional stability over time due to repeated, cyclical application of intermediate pressures to the surface of the blanket. When the thickness of a blanket exceeds the limits of a pressure setting, the pressure during printing becomes insufficient to effect transfer of the ink image from the impression cylinder to the blanket or from the blanket to the paper, or both. Consequently, for a typical blanket, a pressure loss as small as 0.05 to 0.10 mm (0.002 to 0.004 inches) can require a press stop.

Conventionally, the fixed interference described above is accomplished by inserting one or more thin layers of paper or the like between the cloth and the surface of the cloth cylinder to build up the thickness of the cloth. This process is known as packing a cloth. Once the dimensional loss of the cloth reaches a certain amount as described above, additional thickness must be added under the cloth. This involves stopping the press, dismantling the cloth and the original packing, repacking, and then re-fastening and re-tensioning the cloth.

The packing process, however, presents problems in that the process is time consuming, resulting in downtime for the printing equipment. Typically, press downtime can cost several hundred to several thousand dollars per hour. It can take up to 30 minutes to pack or repack a blanket. Time is also lost as the system returns to its optimal setting. In addition, once the wrapping paper is positioned on the cylinder, it can tend to slide, sag and/or fold, making the surface of the blanket uneven and resulting in poor printing results.

To avoid some of the problems associated with packed cloths, press operators, particularly newspaper press operators, have used cloths that do not require packing. So-called "non-packing" cloths have been developed to to provide a specified interference without the need to pack the blanket. No Pack blankets are manufactured with a very precise dimensional accuracy so that they can be installed directly onto a blanket cylinder with the correct amount of interference. These blankets have the advantage of a one-piece construction, which does not require the positioning of packing paper underneath the blanket. This results in minimal downtime for the printing equipment when an old blanket is removed and replaced with a new blanket.

Such unpacked blankets, similar to most printing blankets, are normally constructed of a base material that provides the blanket dimensional stability. Woven blankets are preferred. The base may consist of one or more layers of fabric. The working surface of the blanket that contacts the ink is typically an elastomeric layer of natural or synthetic rubber applied over the base layer or layers. The base layer or layers and the working surface are laminated together using suitable adhesives. Again, such blankets provide some dimensional loss during initial tensioning and installation and continue to lose thickness over their service life. However, once the dimensional loss (thickness) on an unpacked blanket exceeds the limits of the press setting, the blanket becomes unusable without restriction and must be replaced with a new blanket.

U.S. Patent No. 3,652,376 discloses a packing blanket having a compressible layer formed by impregnating a felt material with an elastomeric material. The compressive layer is positioned between the top and bottom layers of non-stretchable reinforced fabrics, which may also be impregnated with an elastomer. The layers are bonded together to form the packing blanket, which is then cured by pressing. This patent does not teach a printing blanket comprising a printing surface layer, and accordingly the patent is not directed to solving the problem of maintaining the dimensional stability of a printing blanket.

Accordingly, there remains a need in the art for a printing blanket that resists dimensional stability loss throughout its lifetime. Such a blanket would reduce expensive downtime for press operators and require few adjustments. of the press during operation.

Summary of the invention

The present invention meets this need by providing a printing blanket that resists dimensional stability loss during its lifetime when subjected to nip pressures. According to one aspect of the invention, a printing blanket is provided that includes at least one printing surface layer and at least one reinforcing woven fabric ply. The blanket may also include a compressible layer as is known in the art. Typically, a printing blanket will include more than one reinforcing fabric ply. To achieve the benefits of the present invention, at least one, and preferably all, of the reinforcing woven fabric plies in the blanket construction should be impregnated with an elastomeric compound prior to incorporation into a printing blanket. The elastomeric compound at least partially penetrates into air spaces of the individual fiber bundles in the woven fabric plies and fixes the fibers against relative movement. Accordingly, the blanket resists permanent deformation when subjected to nip pressures so that the blanket retains at least 95% of its original dimensional stability throughout the life of the blanket. Typically, such useful life may be in excess of one million impressions.

The present invention encompasses the use of fabrics containing both synthetic and natural fibers, as well as blends thereof. Typical fabric compositions include cotton, rayon, polyester, Kevlar (trademark of E.I. duPont de Nemours for a polyamide fiber), and Tencel (trademark of Courtaulds Fibers Inc.). Such fabrics are typically made from yarns or threads having a number of individual fibers twisted into a fiber bundle.

In one embodiment, from about 6 to about 125 gm/m², and preferably from about 10 to 25 gm/m², of the elastomeric compound is impregnated into the woven fabric ply. By impregnated is meant that the elastomer is forced into individual yarns of the fabric. The excess elastomer can be removed, leaving elastomer only within the individual yarn threads. Impregnation should be a mere coating of the surfaces of the fabric. The elastomer compound is selected from the group consisting of natural and synthetic rubbers, latex, thermoplastic polymer resins. Preferred elastomers include carboxylated nitrile rubber and acrylonitrile butadiene rubber.

To impregnate the woven fabric insert, the insert can be immersed in a solution of either a liquid rubber compound or a water-based latex rubber, the solution being forced or squeezed into the insert and the excess solution removed. The liquid rubber immersion solution can be formed by dissolving a solid elastomer in a solvent. An example of a liquid rubber in a solvent suitable for use in the present invention is acrylonitrile butadiene rubber in toluene or methyl ethyl ketone. Alternatively, the elastomer compound can be a water-based latex into which the woven fabric insert can be immersed.

In any event, it is preferred that the woven fabric ply be maintained under tension during the dipping process. Preferably, a tension of about 2.8 to about 16.8 kg/cm of fabric width is maintained. The ply is then dried to evaporate solvent or water, as the case may be, and to solidify the impregnated elastomer.

In another embodiment of the invention, the printing blanket of the present invention can be made to resist permanent deformation by calendering the woven fabric ply with heated rollers to permanently reduce its thickness after impregnation of about 2 to 7 gm/m² of a liquid elastomer, but prior to its incorporation into the blanket. A preferred method of reducing the initial thickness of the woven fabric ply is after impregnation of about 2 to 7 gm/m² of a liquid elastomer, to pass the fabric ply between rotating rollers which have been heated, preferably to a temperature above the boiling point of water, but below that which will scorch the fabric, and which have been adjusted to apply pressure to the fabric. Typical rolling temperatures are in the Range from about 65º to 180ºC (about 150º to 350ºF) and typical pressures applied range from about 2 to 10 bar. The dimension of the insert is reduced, preferably to about 65-75% of its initial thickness, by the calendering step.

Alternatively, the step of impregnating the woven fabric ply may comprise heating the elastomeric compound until it is pliable and then forcing the heated elastomeric compound under pressure into the interstices between the warp and weft yarns of the woven fabric ply. This may be accomplished by spreading the pliable elastomer on one of two rotating rollers and passing the fabric ply between them. By operating the rollers at different speeds, the resulting friction rubs the pliable elastomer into the interstices of the fabric weave and partially into the surface of the individual fiber bundles.

Accordingly, it is a feature of the present invention to provide a printing blanket that resists dimensional loss throughout its life when subjected to nip pressures. This and other features and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings and the appended claims.

Short description of the drawings

Fig. 1 shows a perspective view of a segment of a printing blanket, illustrating the various inserts that make up the blanket.

Detailed description of the preferred embodiments

A typical printing blanket uses a plurality of layers of reinforcing fabric laminated together with an elastomeric adhesive, over which is positioned a thin layer of elastomeric face rubber which acts as the ink carrying and transferring layer. Optionally, such blankets may also contain a thin layer of a microporous elastomeric material which is compressible when subjected to press nip pressures, but recovers to its original thickness when the pressure is released.

As shown in Figure 1, a typical printing blanket 11 includes a base ply 12 of woven fabric. As is conventional in the art, an additional ply or plies of reinforcing fabric, such as plies 14 and 17, are also included in the laminate construction. Typically, the fabric plies are low stretch fabrics, such as certain types of cotton, rayon, polyester or glass. In the normal manufacturing process, the plies are formed from a long web of fabric. The plies 12 and 14 may be bonded together by an adhesive layer 13, which may be a neoprene rubber cement or other suitable adhesive material. The cloth may optionally comprise a layer of microporous compressible elastomeric material 15 disposed beneath the surface layer 18, between any of the subsequent layers of reinforcing fabric, or may be included in a reinforcing layer or between the pressure surface layer and a fabric reinforcing layer.

The surface layer 18 adapted to receive a colored image from a printing plate is typically formed of any suitable polymeric material including both natural rubbers and synthetic resins. For example, some rubber compounds that have been used to form printing blanket surface layers include acrylonitrile butadiene rubber, isobutylene isoprene elastomer, polysulfide rubber, ethylene propylene diethylene terpolymer, natural rubber, styrene butadiene rubber, and a blend of acrylonitrile butadiene and polysulfide rubber. The surface may be smoothed by tallow coating, ground after vulcanization, or textured by any known process such as that disclosed in U.S. Patent No. 4,751,127, assigned to the assignee of the present invention.

The surface layer 18 may be coated in a conventional manner by mixing an unvulcanized rubber compound in a suitable solvent and thereafter knife coating the solution onto a fabric carcass, such as a fabric layer 17. Typically, the application is made in a multiple thin coatings. After each coating is applied, the solvent is allowed to evaporate so that the resulting rubber layer is substantially solvent-free.

The printing blanket 11 is then heated for a suitable period of time to vulcanize or cure the unvulcanized rubber in the structure. The blanket 11 may be initially formed in a long roll or the like from which individual blankets may be cut to size.

It has been determined that within this type of blanket construction, the layer or layers most likely to be permanently deformed by the printing process are the reinforcing fabric layers. Permanent deformation, in which a permanent loss of dimensional stability occurs, is distinguished from temporary deformation, which all printing blankets undergo when subjected to nip pressures. A fabric used in printing blankets is typically constructed of warp yarns woven together in an alternating pattern above and below with weft yarns at an angle of approximately 90º. Both the warp and weft yarns are constructed of numerous, individual fibers twisted into a loose fiber bundle with up to 50% air space in each yarn bundle.

When the individual yarn strands are subjected to repeated, cyclic compression (typically from a few hundred thousand to over a million cycles), the individual yarn strands move, compressing to fill the air space in each bundle. The air originally present in the fiber bundles eventually escapes from the fabric structure. As a result, over time, each fiber bundle deforms and flattens or becomes smaller in cross-section. The resulting result across the entire fabric is a loss of dimensional stability (thickness) of the fabric, which over time exceeds the ability of the press operator to adjust. For a packed fabric, such a loss of dimensional stability requires stopping the press and repacking the fabric. For an unpacked fabric, such a loss of dimensional stability requires replacement of the fabric with a new fabric.

The present invention provides various methods by which dimensional stability loss in reinforcing fabric inserts in a printing blanket can be minimized, thus increasing the useful life of the blanket. In one embodiment of the invention, the yarn fibers in the fabric are made resistant to permanent Deformation is made by impregnating the fiber bundles with an elastomeric compound. The elastomer penetrates and fills the air spaces of the fiber bundles and fixes the relative positions of the fibers. A simple coating of the fabric with an elastomer will be sufficient to create a fabric that has resistance to permanent deformation if the air spaces in the fiber bundles are left. The elastomer must at least partially, if not completely, penetrate the air spaces within individual fiber bundles and fix the fibers against movement.

It has been found that for embodiments of the invention where the fabric is not permanently dimensional reduced prior to use, the minimum amount of elastomer required to achieve press improvements in resistance to permanent deformation is about 6 gm/m² for elastomers having a specific gravity in the range of about 1.0 to 1.4. For the embodiment of the invention where the fabric is permanently dimensional reduced by calendering, it has been found that the minimum amount of elastomer required to achieve press improvements in resistance to permanent deformation is about 2 gm/m².

Appropriate adjustments to this minimum amount can be made for elastomers having specific gravities falling outside this range. The upper limit on the amount of elastomer is that amount which completely saturates and fills all air spaces within the fiber bundles. Again, based on an assumed specific gravity in the range of about 1.0 to 1.4, the maximum amount of elastomer will be about 125 gm/m2. When operating near this upper limit, the degree of stiffness of the fabric must be taken into account. The use of larger amounts of elastomer produces a fabric having greater stiffness. While it is desirable to make the entire fabric layers in the fabric construction resistant to permanent deformation, the benefits of the present invention can be realized in part by such treatment of a single fabric layer.

The elastomeric compound used to impregnate the fabric is selected from the group consisting of natural and synthetic rubbers, such as acrylonitrile butadiene, carboxylated acrylonitrile butadiene or neoprene, and latex rubbers. Preferred elastomers include carboxylated nitrile rubber and acrylonitrile butadiene rubber.

To effect impregnation of the woven fabric insert, the insert may be immersed in a solution of either a liquid rubber compound or a water-based latex rubber, the solution being forced or squeezed into the individual fabric threads and the excess solution removed. The liquid rubber immersion solution may be formed by dissolving a solid elastomer in a solvent such as toluene or methyl ethyl ketone. For example, about 10-20% by weight of elastomer may be dissolved in a solvent. An example of a liquid rubber in a solvent suitable for use in the present invention ranges from about 10-15% by weight of acrylonitrile butadiene rubber in toluene. Alternatively, the elastomer compound may be a water-based latex into which the woven fabric insert may be immersed. The dipping solution has a relatively low viscosity in the range of approximately 2,000 to 20,000 centipoise. This low viscosity allows the liquid rubber or latex solution to penetrate into the individual fiber bundles in the fabric braid. The dipping device can be any suitable container in which the dipping solution is contained. The fabric insert can be passed through the dipping device using conventional powered unwinding and winding rollers.

In any event, it is preferred that the woven fabric ply be maintained under tension during the dipping process. Preferably, a tension of from about 2.8 to about 16.8 kg/cm of fabric width is maintained. This can be easily accomplished using a tension-brake unwind and rolling up of the fabric ply. The fabric ply is then dried to evaporate solvent or water, as may be necessary to solidify the impregnated elastomer.

In another embodiment of the invention, the printing blanket of the present invention can be made to resist permanent deformation by calendering the woven fabric ply with heated rollers and to permanently reduce its thickness after impregnation with a liquid elastomer, but prior to its incorporation into the blanket. In this way, the fiber bundles are permanently compacted before the blanket is built up and fixed in position by the elastomer. Previously precalendered fabric plies that have swollen in blankets have regained their thickness when the blankets have been exposed to conventional liquids and solvents present during the pressing process. This has resulted in an increase in dimensional stability in the fabric ply or plies back to their precalendered thickness. Using the process of the present invention, the fabric is permanently consolidated and is impermeable to liquids and other solvents present during pressing operations.

A preferred method of reducing the initial dimensional stability of the woven fabric ply is after dipping as described above using from about 2 to about 7 gm/m2 of a liquid elastomer to pass it between rotating rollers heated to a temperature higher than the boiling point of water, but less than that which would scorch the fabric, and adjusted to apply pressure to the fabric. Preferably, the dimension of the ply is reduced to about 75% of its initial thickness by the calendering step, and may be further reduced to about 65% of its initial thickness.

Alternatively, the step of impregnating the woven fabric insert may comprise heating the elastomeric compound until it is pliable and then forcing the heated elastomeric compound under pressure into the outer fibers in the individual fiber bundles in the fabric and into the spaces between the warp and weft threads of the woven fabric insert. This may be accomplished by spreading the pliable elastomer on one of two rotating rollers and passing the fabric between the rollers. By operating the rollers at different speeds, the resulting friction stretches the pliable or elastic elastomer into the fabric of the insert and at least partially into the surface of the individual fiber bundles.

Accordingly, in a preferred method of operation, the fabric is passed between two rotating rollers, one roller being driven at a speed that controls the surface speed of the fabric, and the other roller being coated with a compliant elastomer and being driven at a slightly higher speed to create the frictional scrubbing action to force the elastomer into the surface of the fabric and into the spaces between the warp and weft threads. Again, only one surface coating of the fabric with an elastomer will be insufficient to achieve a fabric that has resistance to permanent deformation.

Optionally, the fabric can be passed through the rollers a second time to expose the other side of the fabric to the elastomer.

The elastomer acts to fix the individual fibers in place and prevent movement. Additionally, because the elastomer is forced into only the spaces between yarns and into the outer fibers of individual bundles, the fabric remains relatively more flexible than with full impregnation. The inner fibers in the bundles are left free to move, deflect and bend as needed to secure the cloth to a cylinder.

In order that the invention may be more readily understood, reference is made to the following examples, which are intended to illustrate the invention but are not intended to be limiting of its scope.

example 1

Elastomer impregnated printing blankets were prepared by dipping a typical square woven fabric with two interlining yarns twisted at 8-20 turns per inch into a liquid solution containing 12-15% by weight acrylonitrile butadiene rubber in a solvent and then drying the fabric. A total of 14-28 gm/m² of the elastomer was impregnated into the fabric. The impregnated The fabric obtained was then used to form the backing layers of four interlining blankets. The blankets were manufactured to have an original gauge of 2.08 mm (0.082 inches). The blankets were then installed on two newsprint presses and operated for two months. During this time, the gauge of the blankets was measured periodically on both the drive and operator sides of the presses. After two months and nearly five million impressions, the measured gauge of the blankets was 1.98 mm (0.078 inches), which was slightly more than 95% of the original gauge.

Example 2

Printing blankets were prepared by heating an acrylonitrile butadiene elastomer until it was pliable and coating it on the first pair of rollers operating at different speeds. A textile fabric as in Example 1 was passed between the rollers and the compliant elastomer was slipped into the interstices of the textile fabric. The fabric was then cooled to harden the elastomer. The treated fabric was then used to form the reinforcing layers of four interlining printing blankets. The blankets were prepared to have an original gauge of 2.08 mm (0.082 inches). The blankets were then installed on two newsprint presses and operated for two months. During this time the gauge of the blankets was periodically measured on the drive and operator sides of the presses. After two months and nearly five million impressions, the measured dimension of the cloths was 1.98 mm (0.078 inches), which was slightly more than 95% of the original dimension.

Example 3

Elastomer impregnated printing blankets were prepared by immersing textile fabric as in Example 1 in a liquid solution of 15% by weight acrylonitrile butadiene rubber in toluene and then drying the fabric. A total of 14-28 gm/m² of elastomer was impregnated into the fabric. The impregnated fabric was then calendered between heated rollers to reduce the thickness of the interlinings and the interlinings were used to form the reinforcing layers of four interlining printing blankets. The blankets were prepared to have an original nal gauge of 1.95 mm. Commercial printing blankets manufactured by Day International Corporation under the designation 9500 were used for comparison purposes. The blankets were the same in all respects except for the use of elastomer impregnated fabric and heat calendered elastomer impregnated fabric in the test blankets.

A number of both types of cloths were then installed and operated on newsprint presses. During this time the gauge of the cloths was measured periodically on both the drive and operator sides of the presses, as well as with a cylinder gap of 100 mm and on the opposite side of the cloth cylinder. After 500,000 to two million impressions, the average measured gauge of the untreated cloths was 1.86 mm and 1.80 mm respectively, representing a final thickness that was approximately 92% of the original gauge and a final thickness that was 0.15 mm less than the original. By comparing the treated cloths after 500,000 and two million impressions, an average dimension of 1.91 mm and 1.88 mm was measured, respectively, representing a final thickness that was approximately 96% of the original dimension and a final thickness that was 0.07 mm less than the original.

As a result, the dimensional loss of the elastomer-treated blankets was less than half of the dimensional loss of the conventional blankets. In actual printing operations, this represented the difference between a used blanket and one that could be reused by the press operator.

Claims (17)

1. A printing blanket with improved dimensional stability, wherein the blanket contains a printing surface layer and a reinforcing woven fabric insert, the fabric insert has been impregnated with an elastomeric compound prior to its incorporation into the printing blanket, said elastomeric compound at least partially penetrating air spaces of individual fiber bundles of said fabric insert and fixing said fibers against relative movement, and wherein said blanket resists permanent deformation when subjected to nip printing, such that said blanket retains at least 95% of its original dimension over the useful life of the printing blanket. 2. The printing blanket of claim 1, wherein said fabric insert has been impregnated with from about 6 to about 125 gm/m² of said elastomeric compound. 3. A printing blanket according to claim 2, wherein said elastomeric compound is selected from the group consisting of natural and synthetic rubbers or latexes. 4. A printing blanket according to claim 2, wherein each of said woven fabric inserts has been impregnated with from about 10 to about 25 gm/m² of said elastomeric compound. 5. A printing blanket according to claim 1, wherein: each of said woven fabric inserts has been impregnated with from about 2 to 7 gm/m² of said elastomeric compound and then calendered with heated rollers to permanently reduce its thickness prior to its incorporation into said blanket. 6. A printing blanket according to claim 1, wherein said elastomeric compound has been impregnated into small interstices of said woven fabric insert and at least partially into the surface of individual fiber bundles of said woven fabric insert. 7. A method for producing a printing blanket which contains a reinforcing woven fabric insert and a printing surface layer arranged thereon, said printing blanket having improved dimensional stability, comprising the following process steps: Impregnating an elastomer compound into this woven fabric insert such that this elastomer at least partially penetrates into air spaces of individual fiber bundles of this woven fabric insert and fixes these fibers against relative movement; and Incorporating said woven fabric interlining and said printing surface layer to produce said printing blanket which resists permanent deformation when subjected to nip printing such that said blanket retains at least 95% of an original dimension over the useful life of the printing blanket. 8. The method of claim 7 wherein from about 6 to about 125 gm/m² of said elastomeric compound is impregnated into said woven fabric insert. 9. The method of claim 8, wherein said impregnation step includes the steps of dissolving said elastomeric compound in a compatible solvent to produce a liquid rubber, immersing said woven fabric insert in said liquid rubber, and removing said solvent to solidify said rubber. 10. The method of claim 9, wherein the woven fabric insert is held under tension during the dipping step. 11. The method of claim 10, wherein said woven fabric interlining is held at a tension of about 2.8 to about 16.8 kg/cm with respect to the width of said fabric. 12. A method according to claim 9, in which the woven fabric insert is compressed to remove excess liquid rubber. 13. The method of claim 9, wherein said elastomer compound is selected from the group consisting of natural and synthetic rubbers, including nitrile rubber. 14. A method according to claim 7, wherein the impregnation step includes the steps of providing a water-based liquid latex, immersing said woven fabric insert in said liquid Latex and the evaporation of water from that latex to solidify that latex. 15. The method of claim 7 including the steps of impregnating about 2 to 7 gm/m² of said elastomeric compound into said woven fabric insert and then calendering said woven fabric insert with heated rollers to permanently reduce its mass. 16. The method of claim 15, wherein the calendering step reduces the dimension of said woven fabric ply to about 75% of its initial dimension. 17. The method of claim 8, wherein said impregnation step includes heating said elastomeric compound and introducing said heated elastomeric compound under pressure into the small interstices of said woven fabric ply.